It is known that a body traversed by a light radiation can introduce variations in the state of polarization of such a radiation; to characterize the body from its optical-property standpoint, the knowledge of the state of polarization of the radiation outgoing from the body is of importance. This knowledge is essential when exploiting interference or beating among radiations, since these phenomena occur ony when the radiations have the same polarization. Beside the well known applications of classical optics, optical coherent or heterodyne telecommunications (based on a beating) can be cited, as well as optical fiber sensors or gyroscopes, requiring the use of fibers maintaining a determined state of polarization.
A polarized radiation can be characterized by the electromagnetic field components in an orthogonal reference system x, y. Taking into account only the electrical field, the two components are: EQU E.sub.x =a.sub.1 cos .omega.t EQU E.sub.y =a.sub.2 cos (.omega.t+.phi.)
where a.sub.1, a.sub.2 are the amplitudes of the two components and .phi. is the relative phase. To determine the state of polarization it is necessary to measure the ratio a2/a1 between the two amplitudes and phase .phi., whose sign defines the rotation direction on the polarization image (described on plane Ex, Ey, as t varies). It is also to be taken into account that the state of polarization can vary in time: this usually occurs in systems using optical waveguides, owing to variable mechanical and thermal stresses which modify the optical properties of the transmitting medium.
Devices are already known for the measurement of the state of polarization of a beam under non-stationary conditions. An example is described by R. Ulrich in the paper entitled "Active Stabilization of Polarization on Single-Mode Fiber" presented at the Optical Communication Conference, Amsterdam, Sept. 17-19, 1979 and published at pages 10.3-1 and ff. of the conference records. In this known device, the state of polarization at a fiber output is measured and compared with a desired state, for polarization stabilization purposes. For measuring the actual state a small fraction of the beam emerging from the fiber is extracted by a beam splitter and split into two nearly equal parts. One of these parts is passed through a .lambda./4 plate, split again into two parts, thus allowing the analysis of the left/right circular components; the other too is split into two parts and is used to analyse the .+-.45.degree. linear components. The two pairs of beams thus obtained are sent to two pairs of detectors whose output signals are processed in analog circuits supplying on the so-called Poincare sphere the coordinates of the state of polarization, which depend in a known way on the above-cited parameters.
This known device has the disadvantage of presenting rather slow and imprecise processing circuits and gives rise to receivers sensitivity problems, as phase measurements are made as intensity measurements (direct detection) and under particular conditions they can produce severe angular errors.